1. Field of the Invention
The present invention generally relates to a method and system for the production of submicron materials, and in particular to a method and system of synthesizing, in bulk quantities, nanosized powders, including nanocrystalline ceramics. Even more particularly, the present invention relates to a method and system for increasing the rate and thereby reducing the cost of the production of bulk quantities of nanosized powders by electrothermal gun synthesis.
2. Description of Related Art
Ceramic materials are used in a wide variety of applications, and generally have excellent heat resistance, corrosion resistance, and abrasion resistance, as well as unique electrical and optical properties. Ceramic material, as used herein, generally refers to an oxide, nitride, boride or carbide of a metal, or a mixture thereof. Very fine ceramic powders are used in a large number of industrial processes to introduce or modify material properties. These materials can pose difficulties in sintering but, when they are converted to ultrafine particles, particularly submicron crystalline particles, numerous traditional problems are avoided. Accordingly, several processes have been devised for fabricating ultrafine, or submicron, crystalline materials, such as those of 1-500 nanometer size, referred to herein as xe2x80x9cnanosized,xe2x80x9d xe2x80x9cnanocrystalline,xe2x80x9d xe2x80x9cnanoparticles,xe2x80x9d and the like.
Techniques for producing nanocrystalline materials generally fall into one of three categories, namely, mechanical processing, chemical processing, or physical (thermal) processing. In mechanical processes, fine powders are commonly made from large particles using crushing techniques such as a high-speed ball mill. There are several disadvantages with this approach. Sometimes metallic powders and highly reactive metals are combined with and subjected to such milling, which can pollute the material with a nanocrystalline alloy. Fragmented powders produced by mechanical processes can also result in particles of inconsistent shapes and sizes, and are often coarse and so not suited for high-performance applications.
With chemical processes, nanocrystalline materials are created from a reaction that precipitates particles of varying sizes and shapes, using a family of materials known as organometallics (substances containing combinations of carbon and metals bonded together). It is difficult, however, to produce ultrafine ceramics using organometallics without introducing excess carbon, or nitrogen (or both) into the final composition. Solution-gelation (sol-gel) ceramic production is similar to organometallic processes, but sol-gel materials may be either organic or inorganic. Both approaches involve a high cost of raw materials and capital equipment, limiting their commercial acceptance.
One of the earliest forms of physical, or thermal, processing, involves the formation and collection of nanoparticles through the rapid cooling of a supersaturated vapor (gas phase condensation). See, e.g., U.S. Pat. No. 5,128,081. In that example, a raw metallic material is evaporated into a chamber and raised to very high temperatures, and then oxygen is rapidly introduced. See also U.S. Pat. No. 5,851,507, in which a carrier medium is mixed with precursor material which is vaporized and subsequently rapidly quenched.
Thermal processes create the supersaturated vapor in a variety of ways, including laser ablation, plasma torch synthesis, combustion flame, exploding wires, spark erosion, electron beam evaporation, sputtering (ion collision). In laser ablation, a high-energy pulsed laser is focused on a target containing the material to be processed. The high temperature of the resulting plasma (greater than 10,000xc2x0 K) vaporizes the material so quickly that the rest of the source (any carrier and quenching gases) can operate at room temperature. The process is capable of producing a variety of nanocrystalline ceramic powders on the laboratory scale, but it has the great disadvantage of being extremely expensive due to the inherent energy inefficiency of lasers, and so it not available on an industrial scale.
The use of combustion flame and plasma torch to synthesize ceramic powders has advanced more toward commercialization. In both processes, the precursor material can be a solid, liquid or gas prior to injection into the flame or torch, under ambient pressure conditions (the most common precursor state is a solid material). The primary difference between the two processes is that the combustion flame involves the use of an oxidizing or reducing atmosphere, while the plasma torch uses an inert gas atmosphere. Each of these processes requires relatively expensive precursor chemicals, such as TiCl4 for the production of TiO2 by the flame process, or TiC and TiB2 by the plasma process. A feature of both methods is the highly agglomerated state of the as-synthesized nanocrystalline ceramic powders. While for many applications the agglomeration of the powders is of little significance, there are situations where it is a shortcoming. Loosely agglomerated nanoparticle powders are produced in the combustion flame method of U.S. Pat. No. 5,876,683.
In the plasma process, reactants or feed materials are delivered to a plasma jet produced by a plasma torch. See generally, U.S. Pat. Nos. 4,642,207 and 5,486,675. Alternatively, the feed material may be delivered to the plasma stream by arc vaporization of the anode. The anode is normally metallic but may be a metal-ceramic composite.
An improved plasma torch process is described in U.S. Pat. No. 5,514,349. That process can produce non-agglomerated ceramic nanocrystalline powders starting from metalorganic precursors, and uses rapid thermal decomposition of a precursor/carrier gas stream in a hot tubular reactor combined with rapid condensation of the product particle species on a cold substrate. Plasma torch processes, while gaining some limited commercial acceptance, are still energy inefficient and often involve materials which are extraneous to the products being produced. For example, in the ""349 patent, a working gas must be heated by the plasma arc, which is wasted energy. Also, since the product particles are suspended in the hot process gas stream, it is necessary to quench not just the particles but the process stream as well. The multiple gases used (the reaction gas, quench gas, and passivating gas) are either wasted, or must be separated for reuse.
Another apparatus for producing nanosized particles is the exploding wire device. A conventional exploding wire device is illustrated in FIG. 5. A pulsed current discharge is driven through a small diameter wire that is typically on the order of 0.1 mm. The resulting joule heating of the wire vaporizes and ionizes the wire resulting in plasma which expands radially. The nearly cylindrical plasma contact surface which expands into the ambient gas, undergoes a reaction with the gas and subsequently cools. The mixing process relies on molecular diffusion and the cooling process is nearly isentropic. Production rates are typically only a few milligrams owning to the small diameter wire necessary for complete vaporization.
Another device that builds on the exploding wire method is a conventional capillary plasma device, shown in FIG. 6. Conventional capillary plasma devices produce hydrocarbon-based plasmas that are used to ignite explosives because the plasma will not negatively react with the explosive. The capillary plasma device consists of two non-eroding electrodes positioned at the ends of a non-conducting bore with one open end. A fuse wire is connected between the electrodes. The process begins with an electrical discharge that explosively vaporizes and ionizes the fuse wire between the two electrodes. The discharge is maintained by the erosion and subsequent ionization of the liner to produce a dense plasma inside the bore which then exits from the open end of the device. The sealed end or breech of the gun must be capable of withstanding extremely high pressures in the order of 10,000 psi. The plasma exits the gun and is then used to ignite the explosive. The length to diameter ratio of the bore, L/D, is large which helps to promote bore wall erosion to sustain the discharge. This process is not well suited for ceramic production because the erosion of a ceramic bore would not produce the ionization needed to sustain the plasma and consequently the production would be low.
The electrothermal gun, also known as an electrogun or capillary discharge device (shown in FIG. 7), is a pulsed power device for the production of very high velocity plasma jets and vapors of different metals such as aluminum or titanium. A pulsed, high current arc is struck down the barrel of a gun, between an electrode in the breech of the gun and an electrode at the muzzle. This arc produces rapid vaporization of the electrodes and of the barrel of the gun so that a pulsed, high-temperature, high-velocity plasma jet is fired out the muzzle.
Electrothermal guns are superficially similar to plasma torches. In a plasma torch, an electric arc is used to convert a gas stream into a plasma jet. Electrothermal guns have been used for a variety of applications such as laboratory plasma producers, spacecraft thrusters, propellant ignitors for high performance guns, and plasma armature sources for electromagnetic rail guns. Electrothermal guns also have been used to vaporize and project an initially liquid gas (such as argon) to atomize an external stream of molten metal. In addition, electrothermal guns have been used to heat and propel powdered material for the application of coatings. xe2x80x9cElectrothermal gun synthesisxe2x80x9d is the use of an electrothermal gun to synthesize nanomaterials such as powdered material with particle sizes of approximately 100 nanometers or less. The plasma jet itself is converted physically or physically and chemically into ultra-fine powder. The material composing the plasma jet typically comes from the breech electrode or the muzzle electrode.
In operation, it is frequently necessary to change the breech electrode of the gun. For production of bulk quantities, it is an economic necessity to accomplish this change automatically rather than manually. Thus, simplification of the breech electrode replacement process produces significant cost reduction. For proper operation, the breech electrode must form an effective seal against the reverse flow of high-pressure plasma. The breech electrode is installed with a high-force mechanical preload to insure that the seal remains effective against the sudden onset of high-pressure plasma. In addition, a heavy-duty electrical connection to the breech electrode must be effected. A final consideration is that when the breech electrode is changed, the stub end of the breech electrode is wasted. A method and system for increasing the production rate and reducing the cost of the production of bulk quantities of nanosized powders by electrothermal gun synthesis is needed to overcome these limitations.
The limitations of the prior art are overcome by a new process for the synthesizing nanosized powders. The process utilizes a xe2x80x9chybrid exploding wirexe2x80x9d (HEW) device containing a small diameter solid metal fuse wire or foil sheath inside a cylindrical tube. The ends of the fuse are connected to electrodes which are designed to have sufficient erosion to maintain the heavy metal plasma. The cylindrical tube is designed to contain the large pressures generated when the plasma is produced. The bore may be made of but is not limited to the corresponding ceramics to be produced. Microcrystalline powder of corresponding ceramic may also be retained within the bore.
The process is initiated with an electrical discharge that vaporizes and ionizes the fuse. The tube confines the radial expansion of the plasma which forces it to exit from both ends of the tube where it mixes and reacts with a suitable gas to form nanoscale particles. In addition, the plasma gas will ablate and vaporize the bore wall contributing to the nanoceramic synthesis.
The benefits of the current invention are both chemical, via reaction of the metal vapor with the reaction gas, and physical via ablation of the bore wall, conversion processes are employed to produce the nanocrystalline ceramics. Having the plasma exit both ends of the bore eliminates the problems associated with high pressure breech seals and eliminates the corresponding forces because the internal pressures are used to balance each other. The dual bore also allows increased production by simultaneously operating two guns.
In an alternate embodiment, the fuse wire is replaced with a thin conductive sheath. The conductive sheath fits inside the bore which has been undercut to form an annular area. The annular area is filled with micron sized particles of the desired compound, reactant metal, or combination thereof. The process is initiated as described earlier and the conductive sheath is instantly vaporized. The micron particles are exposed to the plasma where they are vaporized and ejected with the plasma. The plasma jet with the ceramic vapor is rapidly cooled and nanosized particles are formed. The large surface area to volume ratio of the microparticle material overcomes the problems with low production rates by allowing quick and efficient conversions to nanoparticle ceramics.
In another embodiment, a consumable metal insert is placed in the center of the bore. In this version, a spark progresses from one electrode to the metal insert, and then from the insert to the other electrode. The metal insert is eroded and/or vaporized to sustain the discharge and increase production of the nanosized material. The external electrodes can be made of tungsten in an effort to minimize erosion so that they do not have to be replaced. Because the metal insert is in the center, the forces on it are symmetrical and they will balance one another. This eliminates the need for high pressure seals. In addition, there is no electrical contact on the metal insert so that it can be replaced quickly and easily, thus facilitating higher rates of production over prior art methods.
It is one object of the present invention to expedite changing the breech electrode of an electrothermal gun.
It is another object of the present invention to eliminate material waste by providing for complete consumption of the breech electrode without leaving a stub or heel.
The foregoing objects are achieved by modifying the design of the electrothermal gun to simplify installation of the breech electrode. The modified design is open at both ends, and the breech electrode is replaced by a central electrode. In operation, two opposing electrothermal jets are produced, one exiting from each of the two open ends of the gun. There are no mechanical electrical (i.e., direct) connections to the central electrode. Current is transferred to the central electrode by two arcs, one arc attaching to each end of the central electrode. There is little or no pressure differential between the two ends of the central electrode, so the need to provide a seal is eliminated. Also, if a pressure differential does develop between the two ends of the central electrode, there is no adverse consequence to the leakage of plasma and molten metal across the central electrode.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.